Ticks are major vectors of human and animal diseases and a significant nuisance problem. The American dog tick, Dermacentor variabilis, which is the focus of this proposal is the vector of Rocky Mountain spotted fever in the US, and an excellent model for the studies proposed, in part, because of its large size, ease of rearing and the background research on its development. Critical to disease transmission is host recognition, blood feeding, pheromone detection, mating and reproduction. Also important to minimizing host contact with ticks, is the use of personal repellents. It has long been recognized that olfaction is a critical aspect of these processes, and extensive studies have been conducted on olfaction including functional transcriptomics in other blood feeding arthropods like mosquitoes. However, essentially nothing is known about olfaction at the molecular level in ticks. The alternative to the insect antenna in ticks is the Haller's organ, a unique structure found on the tarsi on the first pair of legs. The Haller's organ is basically a molecular black box, and no transcriptome to this organ is available. In preliminary studies in preparation for this proposal, we have developed transcriptomes and successfully examine global gene expression for the first time in the Dv synganglion during female adult reproduction, developed bioassay methods for RNAi screening in the American dog tick, and optimized methods for RNA extraction from tarsi of Dv. The tarsi consist mostly of hard cuticle with minimal soft tissue, and RNA extraction in high yield with good quality is difficult. We have also developed a new, commercialized (natural) tick repellent, BioUD (more effective than Deet) and are interested in understanding the mechanism of repellency and developing rationale approaches for future repellent discovery. Dr. Sonenshine, a co-PI on this proposal, has also conducted much of the pioneering work on tick pheromones and their role in adult reproduction in Dv. The current proposal is to develop the first transcriptome to a tick Haller's organ and to conduct microarray analysis and RNAi screening to identify targets (odorant binding proteins, receptors, catabolic systems and other proteins) for future mechanistic studies of tick olfaction and the development of next generation control strategies for ticks. This will be achieved by the following approaches: Objective 1: Construct the first 454 transcriptome to the Haller's organ of ticks (the model organism will be Dv) and assign putative functions to the assembled contigs; Objective 2: Develop an Agilent Haller's organ microarray and examine global gene expression in response to tick-host interactions and adult reproduction; and Objective 3: Conduct RNAi screening to identify critical messages important in tick olfaction and development. We expect the work proposed will greatly increase our knowledge of the molecular biology of tick olfaction which is currently in its infancy and stimulate future work on chemoreception in this important arthropod group. Future research will include gustatory receptors in the tick palps and their function in tick development and repellency.